Russia has withdrawn most of its strike aircraft from Syria and has not carried out air strikes in the north of the country this week, the U.S. military says.

"They still have helicopters and some transport aircraft. But what we've seen is that the majority of Russian strike aircraft have left Syria," Colonel Patrick Ryder, a spokesman at the U.S. military's Central Command, said on March 18.

His statement represents the first U.S. acknowledgement that Russia is following through on President Vladimir Putin's order to withdraw most Russian forces on March 14.

U.S. officials previously expressed skepticism about the surprise withdrawal plan, and some observers continue to insist that Russia has not pulled out of Syria.

Ryder said that Russian air strike activity also ceased in the north of Syria this week, though Russian warplanes continue to back Syrian forces fighting against the Islamic State group in the vicinity of the ancient city of Palmyra in southern Syria.

The Russian military has said that its jets are flying as many as 25 sorties a day in support of the Syrian offensive to recapture Palmyra from the terrorist group.